1. Field of the Invention
The present invention relates to a low melting point tin salt of aliphatic monocarboxylic acid that has good solubility in various solvents, a method for producing the same, and a coating liquid containing the tin salt for forming a metal oxide film.
2. Description of the Related Art
Tin salts of aliphatic monocarboxylic acid are used as a catalyst, a material for producing a tin oxide film, a coating material, a stabilizer or the like. When utilizing a tin salt of aliphatic monocarboxylic acid in various applications, the dispersibility of this compound in other materials is important, which is similar to the case of using an aliphatic monocarboxylate comprising a metal other than tin. For example, in order to prepare a resin sheet containing such a tin salt as a stabilizer, a resin, the tin salt and a solvent are mixed together, the mixture is formed into a sheet, and then the solvent is evaporated to form a resin sheet. However, tin salts of aliphatic monocarboxylic acids have a high melting point, and thus, the salt is hardly mixed with the solvent or the resin at ordinary temperature. Therefore, it is difficult to provide the function of the tin salt of aliphatic monocarboxylic acid as a stabilizer uniformly for a long time. Alternatively, when a tin salt of aliphatic monocarboxylic acid is used as a reaction catalyst, the tin salt is hardly mixed with a solvent and other ingredients at ordinary temperature. Therefore, it is difficult to allow the function of the tin salt of aliphatic monocarboxylic acid as a catalyst to be exhibited effectively.
In particular, in recent years, tin salts of aliphatic monocarboxylic acid are utilized as materials for forming tin oxide films that can be used in various fields such as electrodes of electronic devices or optical films. For the production of a tin oxide film by the use of such a tin salt of aliphatic monocarboxylic acid, for example, a method is disclosed in Japanese Laid-Open Patent Publication No. 60-81704, which comprises the process of applying a coating liquid containing a tin carboxylate or a coating liquid containing a tin carboxylate and palladium chloride or chloroplatinic acid onto a substrate to form a film, and baking the same. Japanese Laid-Open Patent Publication No. 60-157109 discloses a method of applying a coating liquid containing a tin carboxylate on a substrate and then performing both irradiation with light having an intensity of 30 mW/cm2 or more and baking so that an oxide tin film is obtained.
In Japanese Laid-Open Patent Publication No. 57-129828, a tin carboxylate is applied onto a surface of a magnetic material and baked so that the magnetic material is coated with a tin oxide film. In Japanese Laid-Open Patent Publication No. 55-62400, a tin carboxylate is applied onto a surface of a substrate and baked so that a tin oxide film is formed on the substrate and this film is used as a transparent filter for X-rays. Moreover, in Japanese Laid-Open Patent Publication No. 60-81704, a solution containing a tin carboxylate and an indium compound is prepared, and this solution is applied onto a surface of a substrate and baked so that an electrode made of an indium-tin oxide (ITO) film is formed on the substrate.
When producing a tin oxide film used as an electrode of an electronic device, an optical film or the like using a tin salt of aliphatic monocarboxylic acid, it is preferable that the tin salt has good solubility in various solvents or solvent mixtures so that a coating liquid containing the tin salt can be applied uniformly on various substrates.
However, in general, the solubility of tin salt of aliphatic monocarboxylic acid, in particular, a tin salt of linear aliphatic monocarboxylic acid in various organic solvents is low. Furthermore, the tin salt can be dissolved in only a limited number of solvents, which makes it difficult to select a solvent having high wettability with respect to a substrate. When using a coating liquid in which such a tin salt of aliphatic monocarboxylic acid is dispersed in a solvent, a uniform coating film cannot be formed when it is applied onto a substrate, and a tin oxide film obtained as a final product by thermal decomposition or other processes becomes non-uniform and opaque. Thus, adequate film characteristics cannot be obtained. The temporal stability of the coating liquid is also poor.
In this way, the tin salt of aliphatic monocarboxylic acid is expected to be used in various applications. But, in general, the tin salt of aliphatic monocarboxylic acid has a high melting point, so that it is difficult to mix the tin salt with a solvent or a resin. Furthermore, the tin salt has poor solubility in various solvents, so that it is difficult to dissolve the tin salt in a desired solvent to use in a desired application, for example, to form a tin oxide film.